Fire alarm sensor units which transmit alarm signals by sensing ultraviolet rays emitted from blazes have been known. Such a type of sensor unit is provided with a discharging element such as a discharge tube for sensing the emission of ultraviolet rays and for activating an alarm system using the discharge current.
In order to keep the discharge tube in its active state ready to discharge, a high voltage of 300 volts or more must be kept applied to the tube, and it is practically difficult to use commercially available dry cells for the power source of the sensor unit.
The fire alarm sensor unit of this type is inherently desired to operate with high sensitivity while consuming less power. Less power consumption will permit a compact design of the whole unit as well as the use of commercially available dry cells. On the other hand, a high-sensitive operation will require a circuit arrangement for generating a high voltage as mentioned above.
Moreover, a sensor circuit arranged to operate with high sensitivity is likely to become unstable because of its possible detection of rays other than those from blazes such as spontaneous radioactive rays. Such malfunction is undesirable for this kind of system which is required to have high-reliability in operation to prevent the occurrence of a malfunctional alarm.
In view of the foregoing technical problems, the present invention is contemplated to solve the problems effectively. The present invention is based on the fact that spontaneous radioactivity and the like causing malfunctional alarms occur intermittently whereas ultraviolet rays are emitted continuously from blazes caused by a fire disaster.